This disclosure generally relates to medical monitoring systems and devices, and more specifically to patient monitors using one or more capacitive electrodes.
Electrocardiograms (ECGs) are graphic depictions of electrical activity in the heart. ECGs are produced by electrocardiographs which are available as stand alone devices, portable devices, and/or as integrated functions in various types of multi-vital sign monitoring devices. ECGs are depicted by time (ms) versus voltage (mV) and typically are represented as a waveform. The typical five important aspects, or portions, of an ECG waveform are the P wave, QRS complex (represented as the combination of the Q, R, and S waves respectively), and T wave. The less frequently seen sixth portion is a U wave. The data produced from the graphical depictions are useful in diagnosis of patients to determine what, if any, and the extent to which heart-related problems exist in a patient. Respiration monitors are also available that use chest electrodes that are similar or identical to ECG electrodes that are similar or identical to ECG electrodes. For example, respiration rate measurement may be determined using impedance pneumography, where a high-frequency AC current is passed between at least two electrodes, including a driving electrode and a receiving electrode, on the patient's chest and an impedance between the electrodes is determined. Respiration is then monitored according to the changing impedance values as the patient breathes.
Both electrocardiographs and respiration monitors must have protection circuitry to protect the electronics of those devices from high voltage exposure due to operation of a defibrillator on a patient to which the monitoring devices are connected. Patients experiencing sudden cardiac arrest are treated with a defibrillation shock to the chest. The defibrillation shock is typically in the range of 3 to 5 kV and 50 amps, and typically lasts between 5 and 20 ms. Such a high voltage and current are necessary in order to stop the patient's heart from unproductive fluttering (fibrillating) and to allow the heart to restart effective pumping of blood. Typically, respiration monitors and electrocardiographs are separate from the defibrillator device, and the chest electrodes and lead wires are connected to the patient when the defibrillator delivers the shock. Thus, the electrocardiograph and respiration monitors must withstand the significant voltage and current of the defibrillation and continue working properly.